In 1991, the H3N2 Influenza Virus Vaccine A strain was changed from A/Shanghai/ 11/87 to A/Beijing/353/89 and the Influenza Virus Vaccine B strain was changed from B/Yamagata/16/88 to B/Panama/45/90 in accordance with the recommendations of the World Health Organization as a consequence of changes in circulating human influenza virus strains in the United States and elsewhere. Purified reagents for the production and standardization of single-radial-immunodiffusion (SRID) potency test antigens and antisera were made to these new strains as well as to several closely related strains. Comparison of reagents prepared to A/Shanghai/16/89 and A/Guangdong/39/89, 2 closely related strains from the previous year, showed that strains which are considered indistinguishable from each other may produce reagents which are virtually interchangeable. This is an important finding since final selection of the vaccine strains often is completed late in the production season, and the use of heterologous reagents can save both money and time. A reassortant virus to A/Beijing/323/89, X-109, showed some antigenic differences from its parent wild type virus. This type of change during reassortment has been seen in the past, and often is reflected by a high avid type reaction to all or most antigens and antisera in the hemagglutination-inhibition panel. Although X-109 was ultimately designated as suitable for vaccine production, changes such as this have raised concerns as to the antigenic appropriateness of such reassortants. Wyeth Laboratories has produced some wild type A/Beijing/353/89 vaccine antigen to be used in clinical and laboratory studies in comparison to the X-109 used for this years vaccine. We will use the information from these studies to aid in the decision process with respect to the use of a reassortant virus with high avid properties. Since these types of reassortants are nearly always high yielding viruses, their inclusion as vaccine strains, with the confidence that they will produce appropriate antibody responses, will help ease the strain of long production seasons. demonstrate any appreciable blocking to date.